So,
Eventhough the truth was told to Abhi, it was told in a situation that said otherwise. If Abhi had witnessed RaBul on his own or found some other proof, he would have taken it differently. But he got to know about RaBul right after his sister's marriage was broken and Purab was with another girl on a mantap and Pragya was there too. And Pragya had just told him the previous night, she had no idea where Purab was even if he asked 1000 times!
Tannu changed Purab's letter, she didn't care even about Aaliyah to tell her the truth about Purab. Tannu also tried to pretty much kill Pragya... But Abhi never gets to see this side of her. Did Abhi get to know Tannu called Sarla and told her the truth? Probably not. He also doesn't know Taaliyah were planning a cheap trick on Puneet.
Purab never bothers to hold Abhi responsible for playing with Pragya's life while he receives beatings and blames.
Pragya does not really try to tell Abhi the truth that Purab kept BB and Aaliyah unaware of each other and BB and her did not know about Aaliyah while BB started loving Purab. Yeah, Abhi never listens to anyone, but Pragya can tell his Daadi to make him understand or somehow get him to figure things out.
So, why Abhi is so oblivious to everything that goes on in Kumkum Bhagya?
I think this is all to build up for his great downfall.
Pragya will leave him for whatever reasons she will have (to let him be happy with Tannu?).
He will lose money (thanks to Taiji/Bhabhi? Corporator might take revenge?).
Tannu, the fair weather girlfriend will ditch him.
Daadi will know how he played around with Pragya and leave him or be seriously disappointed in him?
Aaliyah will just say "I was too sad with a broken heart" and wash her hands off?
And Abhi will be left with no one!
There must be a reason why creatives are not clearing things for Abhi!!!
6